According to basic aerodynamic principles, the forces which act on an aircraft in flight are shown in FIG. 1. These forces include lift, weight, thrust and drag. For straight and level unaccelerated flight, thrust is equal in magnitude but opposite in direction to drag, and weight is equal in magnitude and opposite in direction to lift. Maneuver control over an aircraft in flight is obtained by changing the magnitude or direction of these forces, and to thereby cause the aircraft to change its attitude in pitch, roll or yaw. Not surprisingly, any one aspect of attitude control can not be changed without there being some cross effect on another aspect.
Of the forces which act on an aircraft, the factors which affect thrust, lift and drag are to at least some extent controlled by the pilot. The thrust force is simply changed by operation of the aircraft power plant. On the other hand, lift and drag are aerodynamic in nature and are dependent on the particular configuration of the air vehicle and its air foils. While all of the aerodynamic factors are major consideration for aircraft design, a discussion of the lift force is most helpful here. It is well known that the mathematical expression for lift is dependent on several variables and is: EQU L=1/2 .rho.S V.sup.2 C.sub.l
or EQU L=1/2 .rho.S V.sup.2 C.sub.l.alpha. .alpha.
Where:
L=Lift PA1 .rho.=air density PA1 S=wing area PA1 V=velocity (speed) PA1 C.sub.l =Coefficient of lift for the wing PA1 .alpha.=angle of attack of the wing ( measured from direction of relative wind) PA1 note: C.sub.l.alpha. is the change in C.sub.l with change in .alpha..
From the above expressions, it can be appreciated that the lift force (L) which is generated by an airfoil, or wing, is a function of the airfoil design (S and C.sub.l) as well as the conditions of flight (.rho., V and .alpha.). For the moment, consider the effect different velocities (V) have on the aircraft. In order to maintain the required lift, as V changes the C.sub.l must also change. The coefficient of lift (C.sub.l), however, involves consideration of wing design. It happens that some wing designs are particularly good for creating lift at relatively high speeds. These same wings, however, may be relatively ineffective at lower speeds. On the other hand, some wing designs are well suited for creating lift at lower speeds but are generally ill suited for generating lift at the higher air speeds. Stated differently, no single wing configuration is optimal for both high speed and low speed flight. Nevertheless, it is clearly desirable that an air vehicle be able to fly with effective control at both high and low speeds.
A very important engineering consideration when determining the flight envelope for an aircraft involves the ability of the craft to transition from high speed flight to low speed flight. One solution to this problem has been to create reconfigurable wings. To this end, several mechanisms have been proposed. These mechanisms include such devices as flaps, slots and slats as well as swing wings. In each case, the device is manipulated by the operator (pilot) to alter the configuration of the wing and thereby change its coefficient of lift (C.sub.l). Specifically, as will be appreciated by referring to the lift expressions given above, as the velocity (V) of the aircraft is reduced, an increase in the coefficient of lift (C.sub.l) is necessary to maintain the same lift.
Still referring to the expressions for lift which are given above, it will be appreciated that in addition to a reconfiguration of the wing, the requisite lift to keep the aircraft aloft as it slows down can be generated if the angle of attack (.alpha.) is increased. A change in the angle of attack .alpha. for this purpose, however, is good only up to a point. As all pilots know, at a determinable high angle of attack, an airfoil will stall and will no longer create the lift necessary to keep the aircraft flying. Consequently, considerations for angle of attack (.alpha.) and coefficient of lift (C.sub.l) must be made together.
For drone aircraft, not all of the creature comfort considerations involved in piloted aircraft are involved. Nevertheless, the aerodynamic response of the aircraft is still a major concern. Additionally, as a somewhat competing concern, it is desirable with drone aircraft to use as few different component parts as are necessary to create an efficient air vehicle. It is also desirable to create redundancy wherever possible.
In light of the above, it is an object of the present invention to provide an air vehicle having a flight envelope which has a relatively extended range of operational air speeds that includes both high speed flight and low speed flight. It is another object of the present invention to provide an air vehicle which incorporates aerodynamic lifting and control panels, such as wing tips and tail panels, that are interchangeable with each other in order to minimize and simplify maintenance efforts and provide for parts commonality. Yet another object of the present invention is to provide an air vehicle which is reliable and easily controlled. Still another object of the present invention is to provide an air vehicle which is relatively easy to manufacture, operationally easy to control and comparatively cost effective.